(The above elisp produces a function that inserts a long format date on the key (in this case C-c d), the date + time in short format on C-u C-c d, the short format date on C-u C-u C-c d, and the short format time on C-u C-u C-u C-c d)

You could use this trick to make a 'start-slime' replacement that used clojure by default, but sbcl if you press C-u before the key binding.

+1 Now I finally understand what's meant by "two prefix arguments" (as seen in e.g. the magit command magit-pull). BTW, Xah Lee wrote about this and other possible prefix values at ergoemacs.org/emacs/elisp_universal_argument.html. For instance, C-u- without a number makes prefix equal to the symbol -.
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echristophersonSep 30 '14 at 20:14